Low emitter resistance is demonstrated for AlGaAs/GaAs heterojunction
bipolar transistors using Pd/Ge contacts on a GaAs contact layer. The
contact resistivity to 2-10 x 10(18) cm-3 n-type GaAs is 4-1 x 10(-7)
OMEGA-cm2. These are comparable to contact resistivities obtained with
non-alloyed contacts on InGaAs layers. The non-spiking Pd/Ge contact
demonstrates thermal stability and area independent resistivity suitab
le for scaled devices. The substitution of Pd/Ge for AuGe/Ni GaAs emit
ter and collector contacts reduced by an order of magnitude the emitte
r-base offset voltage at high current densities and increased f(t) by
more than 15% with significantly improved uniformity for devices with
2 and 2.6 mum wide emitters having lengths two, four and six times the
width.